The present invention relates to a brake booster, and more particularly, to a brake booster which allows an increased output to be obtained in a reliable manner when a force of depression of a reduced magnitude is applied to a brake pedal, as would be required in an emergency brake which requires a braking effort of an increased magnitude.
A brake booster is known in the art which allows an output of an increased magnitude to be obtained in response to a small force of depression applied to a brake pedal as in an emergency brake where a braking effort of an increased magnitude is required.
Conventional prior art proposed in the art for a brake booster which functions in this manner comprises either a scheme in which a jumping quantity, which is a rise in the output at the commencement of actuation, is increased to provide an output of an increased magnitude, or a scheme in which a booster ratio is increased to provide an output of an increased magnitude.
In the prior art practice, either one of the described schemes has been used to provide an output of increased magnitude during an emergency brake operation, but there has been a need for a brake booster which provides an increased rise in the output during the emergency brake operation by combining the both schemes.
Accordingly, it is an object of the present invention to provide a booster which is simple in construction and which is capable of increasing the output in immediate response to a quick depression of a brake pedal.
Specifically, the present invention relates to a brake booster comprising a valve body slidably disposed within a shell, a power piston mounted on the valve body, a constant pressure chamber and a variable pressure chamber defined forwardly and rearwardly of the power piston within the shell, a valve mechanism disposed within the valve body and which switchably controls the supply to or discharge from the variable pressure chamber of a fluid, an input shaft for operating the valve mechanism, and a reaction transmitting mechanism for transmitting a brake reaction which acts on an output shaft to the input shaft through the valve mechanism, the valve mechanism including a valve plunger connected to the input shaft and having an atmosphere valve seat, a first vacuum seat formed on the valve body, and a valve element which is adapted to be seated upon the atmosphere valve seat and the first vacuum valve seat. In accordance with the present invention, the brake booster further comprises a tubular member slidably mounted on the valve body and having a second vacuum valve seat which is juxtaposed with the first vacuum valve seat, an inoperative position retaining mechanism for retaining the tubular member at its most advanced position relative to the valve body when the booster is not actuated, and an operating mechanism for releasing the retention of the tubular member by the inoperative position retaining mechanism to allow it to retract relative to the valve body so that the second vacuum valve seat can be seated upon the valve element when the input shaft has moved forward through a given stroke relative to the valve body from its inoperative position, said reaction transmitting mechanism being arranged such that during a normal brake operation where the tubular member assumes its most advanced position relative to the valve body, it transmits a brake reaction to the valve plunger through a first reaction transmission path having a small booster ratio while during an emergency brake operation where the tubular member has retracted through a given stroke relative to the valve body, it transmits a brake reaction to the valve plunger through a second reaction transmission path having a greater booster ratio.
With the described arrangement, during the emergency brake operation where the brake pedal is quickly depressed, the operating mechanism releases the tubular member from the retention relative to the valve body, whereby the tubular member retracts rearward relative to the valve body to allow the second vacuum valve seat to be seated on the valve element. As a consequence, a separation between the atmosphere valve seat and the valve element, or the amount by which the atmosphere valve is opened increases. Consequently, the atmosphere is rapidly introduced into the variable pressure chamber through the atmosphere valve, allowing the output from the brake booster to rise rapidly. In a servo balance condition which is reached subsequently, the valve plunger retracts through an increased stroke relative to the valve body than during a normal brake operation, thus providing a grater jumping quantity than during a normal brake operation. At the same time, the brake reaction is transmitted to the valve plunger through the second reaction transmission path, whereby the output rises with a greater booster ratio than during the normal brake operation. In this manner, it is possible to increase the output from the brake booster in immediate response to a quick depression of a brake pedal. In other words, there is provided a brake booster which meets the need of the prior art with a simple construction.
Above and other objects, features and advantages of the present invention will become apparent from the following description of an embodiment thereof with reference to the attached drawings.